1. Field of the Invention
The present invention is directed toward the field of manufacturing integrated circuits. The invention is more particularly directed toward an improved apparatus and method for controlling the flow of process material into a thin film deposition system.
2. Description of the Related Art
Presently, aluminum is widely employed in integrated circuits as an interconnect, such as plugs and vias. However, higher device densities, faster operating frequencies, and larger die sizes have created a need for a metal with lower resistivity than aluminum to be used in interconnect structures. The lower resistivity of copper makes it an attractive candidate for replacing aluminum.
A well established technique for depositing copper is through the use of chemical vapor deposition ("CVD") processing. For example, chemical vapor deposition of copper is achieved by using a precursor known as Cupraselect.RTM., which has the formula Cu(hfac)L. Cupraselect.RTM. is a registered trademark of Schumacher of Carlsbad, Calif. The Cupraselect.RTM. consists of copper (Cu) bonded to a deposition controlling compound such as (hfac) and a thermal stabilizing compound (L). The (hfac) represents hexafluoroacetylacetonato, and (L) represents a ligand base compound, such as trimethylvinylsilane ("TMVS").
During the CVD of copper using Cu(hfac)L, the precursor is vaporized and flowed into a deposition chamber containing a wafer. In the chamber, the precursor is infused with thermal energy at the wafer's surface. At the desired temperature the following reaction results: EQU 2Cu(hfac)L.fwdarw.Cu+Cu(hfac).sub.2 +2L (Eqn. 1)
resulting copper (Cu) deposits on the upper surface of the wafer. The byproducts of the reaction (i.e., Cu(hfac).sub.2 and (2L)) are purged from the chamber which is maintained at a vacuum during wafer processing.
FIG. 1 illustrates a copper CVD system of the prior art. Specifically, a copper deposition system 90, comprises a deposition chamber 100, pressure control unit 120, a precursor delivery system 130, and a gas delivery system 140. The chamber 100 is defined by sidewalls 102, floor 104 and lid 106. Process gases A and B are introduced to the chamber 100 through a showerhead 108 incorporated into the lid 106. The pressure control unit 120, (e.g., a vacuum pump), is coupled to the process chamber 100 via a valve 122 (e.g., a throttle valve) to control the chamber pressure.
In the precursor delivery system 130, liquid precursor such as Cupraselect.RTM. flows from ampoule 132 through a liquid mass flow controller (LMFC) 134, a valve 136 to a tee 138 via conduction lines 133 and 135. In the gas delivery system, a gas "A" is delivered to the tee 138 from a gas "A" source 142 via a mass flow controller (MFC) 144 and valve 146. The Gas "A", an inert gas such as argon or helium, facilitates the flow of liquid precursor from the tee 138 to the showerhead 108. The gas delivery system 140 also delivers a Gas "B", e.g., Argon, directly to the showerhead 108 via a gas "B" source 141, mass flow controller 143 and valves 145 and 147. In the showerhead 108, the liquid precursor expands into a mist and mixes with gas "B". The showerhead 108 contains a hot plate 115 that is heated by, for example a resistive coil. The precursor mist evaporates upon striking the hot plate 115 and forms a vapor.
The deposition chamber 100 further contains a heated susceptor 112 (workpiece support) for retaining a substrate 116 such as a semiconductor wafer onto which copper is to be deposited. The susceptor 112 is fabricated from a durable metallic material such as aluminum or a ceramic material such as aluminum nitride or boron nitride. The susceptor 112 also contains additional components such as resistive heater coils 113 to generate heat within the substrate support 112 which is conducted to the wafer 116. Copper is deposited onto the substrate 116 by CVD when the vaporized precursor contacts the heated wafer.
One problem associated with using Cupraselect.RTM. for CVD is the complicated delivery process used to couple the material from the liquid storage ampoule to the process chamber 100. In the prior art, the liquid Cupraselect.RTM. is mixed with a gas "A" such as Argon, Helium or any other inert gas between the ampoule 132 and the process chamber 100. To precisely deposit a thin layer of copper on the wafer surface, the flow of Cupraselect.RTM. to the vaporizer must be carefully controlled. In the prior art, a gate valve or isolation valve is needed to create a low pressure space to ensure Cupraselect.RTM. evaporation and fast pump-down through a separate diverter path. Such valves provide for high throughput but, unfortunately, have moving parts and O-ring seals that present a potential risk of mechanical particle generation. The gate valve and gas "A" delivery system also complicate the design and construction of the CVD system 90 and add to its cost. Therefore, the components used to deliver the precursor should be minimized so as to reduce cost and facilitate complete purging of the system when so needed.
Furthermore, in prior art systems, liquid precursor shut-off is problematic due to residual liquid precursor in the line between the tee 138 and the showerhead 108. This residual liquid precursor is continuously being drawn into the chamber 100 resulting in an over-flood of precursor. Consequently, the interior of the chamber 100 and other parts of the apparatus become undesirably coated. The coating subsequently flakes off into large particles which can contaminate the wafer 116 and other parts of the apparatus.
Accordingly, it is desirable to provide an apparatus and method for improved control of a precursor material in a substrate process system to reduce the likelihood of plating or particle formation within the system as well as increase deposition rate.